Process for producing aluminum-coated black iron strip



Sept. 29, 1959 w. F. BUGBEE ETAL 2,906,641

PROCESS FOR PRODUCING ALUMINUM-COATED BLACK IRON STRIP Original Filed June 21, 1954 IT!!! Elm 1 Power Vacuum Supply Pump l6 22 INVENTORS Wat/(arc EBU7Q4 BY Phi/ a pick/(17A PROcEss FOR PRODUCING ALUMINUM-COATED BLACK IRON s'rRIP Wallace F. Bugbee, Lowell, and Philip I. Clongh, Reading, Mass., assignors to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Continuation of application Serial No. 438,318, June 21, 1954. This application April 18,1958, Serial. No. 729,292

1 Claim. 01. 117- 50 This application is a continuation of our copending ap plication Serial No. 438,318,filed June21, 1954, and noW forfeited.

This invention relates to coating and more particularly to coating of black iron for the purposes of producing a tin plate substitute.

The use of aluminum as a substitute for tin in the coating of black iron has long been the objective of numerous workers in the art due to the wide fluctuation in the availability and price of tin, particularly during times of war. Despite the great amount of research which'has been done, it is believed that none of the prior workers has been commercially successful. The principal reason for this has been a lack of appreciation of the fundamental requirements for an adherent, corrosion-resistant coating of aluminum on black iron.

Accordingly, it is a principal object of the present invention to provide an improved process for forming an adherent, corrosion-resistant coating of aluminum on black iron.

Another object of the invention is to provide such a process which will give a highly reflecting or shiny, corrosion-resistant, adherent coating in a'single-stage operation.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the process involving the several steps and the relation and the order of one or more of such steps with respect to each of l atented Sept. 29, 1959 2 r tion at substrate temperatures of 500 F.-700 F; is preferred.

In the coating of black iron for the manufacture of' tin cans, it is highly desirable that the equipment be run continuously. Since the degree of attack between aluminum and carbon, which is preferably employed as a crucible for holding the supply of molten aluminum, is a direct function of the temperature of the aluminum, it is preferred that the molten aluminum be at the lowest possible temperature consistent with an adequate supply of aluminum vapors. Temperatures on the order of 1150 C.1200 C. have been found suitable. Accordingly, in a preferred embodiment of the invention, the

the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claim.

In a preferred embodiment of the invention, the black iron to be coated with a thin aluminum film is first thoroughly cleaned. It is then introduced into a vacuum chamber where a low pressure on the Order of less than 1 micron Hg abs. is maintained. During its passage through the vacuum chamber, the black iron passes over a source of aluminum vapors. While the black iron is being coated, it is also maintained at an elevated temperature. This temperature is preferably less than the annealing temperature of the black iron which is about 700 F. The black iron temperature is greater than a minimum temperature which depends somewhat upon the aluminum deposition rate which is a function of, among other things, the temperature of the source of aluminum vapors. When the source of aluminum vapors is at a high temperature on the order of 1600 C. the black iron temperature should be at least as high as about 450 F.500 F. during coating. However, when the source of aluminum vapors is at a low temperature on the order of 1150 C.1200 C., the black iron can be maintained at a temperature on the order of 400 F.- 500 F. during coating. If the above minimum temperatures are not maintained, the coating will not be adherent. Accordingly, the coated black iron will be worthless as tin can stock unless it is subjected to further treatment to increase its adhesion. To be on the safe side, operamolten aluminum in the source is maintained at a temperature on the order of 1150 C.-1200 C., and the black iron to be coated is maintained at a temperature on the order of 500 F.-700 F. Additionally, the substrate-to-source distance is preferably maintained less than one mean free path. At pressures on the order of 1 micron Hg abs., the mean free path is 2.6 inches.

In addition to the above requirements, it has been found that the time and rate of deposition of the aluminum are extremely important in obtaining a highly reflecting surface. If the aluminum is deposited at a rate less than about 0.01 grams per square foot per second, a time in excess of about 10 seconds is required to give a film having a requisite thickness of 0.1' gram per square foot.

For some reason which is not fully understood, the film deposited over this long time and at this slow rate is milky or hazy and is unsatisfactory (from an esthetic standpoint) as a tin plate substitute. Accordingly, the aluminum deposition rate is preferably kept above about 0.01 gram per square foot per second to assure creation of a shiny aluminum coating. The best method of assuring this high coating rate, when a relatively low temperature (ll50 C.l200 C.) aluminum vapor source is employed, is to provide a large area of molten aluminum. If this area is on the order of 50% to of the area of black iron being coated at any instant of time, then the total time for depositing a requisite thickness of aluminum 0.1 gram to 1 gram per square foot (4 to 30 microinches as calculated by measured resistance) is on the order of less than one second and the aluminum deposition rate is 0.1 gram to 1 gram per square foot per second.

In a preferred embodiment of the invention, the source of aluminum vapors has a dimension across the path of travel of the iron strip which is on the order of the width of the strip. The dimension of the source along the path of travel of the strip is preferably on the order of several inches. When the iron strip passes within a few inches of the source (which is preferred), the area of the source thus becomes about the same size as the area of iron strip being coated. The iron strip is thus coated in less than a second to give an adequately thick aluminum coating of high luster and good adhesion.

While the exact mechanism involved in forming the adherent, highly reflecting, corrosion-resistant bond of the present invention is not completely understood, it is believed to be a diffusion bond. Although this bond is formed at temperatures well below the temperatures ordinarily considered essential for difiusing aluminum into iron, theoretical considerations can explain the formation of a diffusion bond at the temperatures employed. The aluminum approaching the heated iron in the vacuum chamber is present as atoms, and, since the base iron is hot, all of the energy of the atoms can be used for diffusion rather than for heating up the base into which these atoms are to diffuse. The energy existing in the hot iron permits the exceeding of some threshold value so that diffusion can occur at a much lower temperature than is indicated for massive metals. Addi- 3 tionally, since the aluminum is present as atoms, there is no necessity. of overcoming the attractive bonds (Van der Waals force) that are present in solid or massive materials.

Referring now to the drawing which indicates sthe appa ratus used to verify experimentally this invention and referring also to several specific experiments, the limits of the invention as applied to the manufacture of tin plate will be illustrated.

Example I A sheet of black iron 10, with a thickness of 0.0095 inch, was thoroughly cleaned with isopropyl alcohol, dried and positioned in a bell jar 12 in contact with a heating element 14. The bell jar 12 was evacuated to a pressure of less than 0.2 micron Hg abs. by a suitable vacuum pumping system 16 and the black iron sheet 10 was heated to a temperature of 480 F. As a source of aluminum vapors, there wasused a filament 18 having an area of 2 square inches which was spaced 3% inches from the black iron 10 and carried a charge of aluminum. This filament 18 was separated from the black iron 10 by a movable shutter 20 and was heated to a temperature of 1200" C. by an electric current flowing from a suitable power supply 22 to evaporate the aluminum. When the aluminum reached 1200 C., the shutter 20 was moved aside for 2 seconds to deposit a coat of aluminum 4 micro-inches thick (as calculated by resistance measurement) on the black iron 10. The black iron 10 was cooled and then removed from the bell jar. of the aluminum deposit was removed by chemical means to get an accurate measure of the weight of aluminum actually deposited. From this weight (which was 0.041 gram per square foot), the aluminum deposition rate was calculated for this experiment to be 0.02 gram per second per square foot. The resultant aluminum coating had an excellent adhesion and was very shiny.

Example II In this case, the black iron was coated under conditions almost identical to those of Example I except that A portion the substrate was 7% inches away from the source of aluminum vapors so that 15 seconds were required to form an adequately thick coating (0.038 gram per square foot) of aluminum. The resultant coated product had a milky or hazy appearance. The coating rate in this case (as measured by chemical. analysis) was 0.0025 gram per second per square foot.

Example III In this case, the conditions were very similar to Example I except that the black iron temperature was only 300 F. during coating. The aluminum film deposited under these conditions was shiny, but not sufficiently adherent.

Since certain changes may be made in the above process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

In a process for producing an aluminum-coated black iron strip which is suitable for manufacture of containers and the like and wherein said strip of black iron is moved past at least one source of aluminum vapors positioned in an evacuated chamber to deposit a thin film of aluminum on the black iron, the molten aluminum in the source of aluminum vapors being maintained at a temperature on the order of 1100 C. to 1200 C., the improvement which comprises maintaining the black iron at a temperature between a maximum temperature of about 700 F. and a minimum temperature of about 400 F., the area of hot, molten aluminum being sufiiciently large and the source of aluminum. vapors being sufiiciently close to the black iron during coating so that aluminum is deposited on the black iron at a rate in excess of 0.01 gram per square foot per second.

McManus et a1. Aug. 13, 1946 Cook June 1, 1948 

